In the manufacture of microelectronic hardware and other products, automated dispensing apparatus are typically used to dispense small amounts of droplets of a highly viscous material in a non-contact manner onto a substrate or workpiece. Exemplary highly viscous materials include, but are not limited to, greases, lubricants, sealants, epoxies, solder flux, solder paste, adhesives, solder mask, thermal compounds, cyanoacrylates, under-fills, oil, encapsulants, potting compounds, inks, silicones, and other viscous materials. Generally, such highly viscous materials cannot easily flow under their own weight at room temperature.
Conventional automated non-contact dispensing apparatus for viscous materials include an air-operated valve element, such as a needle, reciprocated for selectively engaging a valve seat surrounding a discharge passageway. In a process commonly referred to as jetting, droplets are dispensed by retracting the needle from contact with the valve seat, which allows an amount of the viscous material to flow under pressure from a filled fluid chamber through a gap separating the needle from the valve seat and into the discharge passageway. The needle is then moved rapidly toward the valve seat to close the dispensing apparatus, which causes the amount of viscous material to be forced through the discharge passageway and a comparable amount of the viscous material to be ejected from a discharge orifice of the discharge passageway. The small amount of ejected viscous material is propelled as a droplet toward a workpiece, which is spaced from the discharge outlet. In many industrial applications, it is desirable to apply these highly viscous materials to the workpiece in a controlled and consistent manner. For example, it may be desirable to dispense a specified amount (e.g., by weight or volume) of viscous material onto the workpiece. Fluctuations in material temperature and/or non-optimal operating temperatures may cause undesirable problems during the dispensing of the viscous material.
More particularly, the viscosity of a material is an important property that significantly affects the overall quality and consistency of the dispensing process. Viscosity is generally dependent on temperature, and is typically inversely related to temperature. Thus, as the operating temperature increases, the material typically becomes less viscous, and vice versa. In many dispensing apparatus, the stroke of the needle (e.g., the gap between the needle and valve seat) may be selected to deposit a desired amount of the viscous material onto the workpiece. This may be done, for example, by assuming an ideal dispensing temperature and determining the needle stroke corresponding to the desired amount of material to be dispensed onto the workpiece. If, for example, the actual dispensing temperature is higher than the ideal temperature, the viscosity of the material may be lower than expected and consequently more material may be dispensed onto the workpiece than desired. On the other hand, if the actual dispensing temperature is less than the ideal temperature, the viscosity of the material may be higher than expected and consequently less material may be dispensed onto the substrate than desired. A variation in dispensing temperature of as little as 1-2 degrees Celsius may cause significant changes in the consistency of the dispensing process. Such variations may cause more or less weight or volume of material to be dispensed and may further affect the coverage area and edge definition of the droplet on the workpiece.
In a conventional dispensing apparatus, a heater is typically positioned adjacent the tip or dispensing orifice of the apparatus for heating the viscous material prior to dispensing an amount onto the workpiece. In many situations, however, the heater is not capable of bringing the viscous material up to its ideal dispensing temperature before being dispensed therefrom. This may be due, for example, to high flow rates of the viscous material through the apparatus, the thermal properties of the material, including thermal conductivity, specific heat, etc., or other factors. In any event, and as discussed above, the inability to reach or sustain the ideal dispensing temperature may significantly affect the quality (e.g., weight, volume, edge definition, etc.) of the dispensed liquid.
Some prior dispensing systems have incorporated auxiliary heaters for heating the viscous material prior to its delivery to the dispensing apparatus. By way of example, in some applications an in-line heater may be positioned in the fluid conduit line that feeds the viscous material to the dispensing apparatus. In other applications, such as hot melt adhesives, the fluid conduit line may be configured as a heated hose. In these applications, however, there is typically a cold junction between the fluid conduit line and the dispensing apparatus that results in temperature variations in the viscous material. Moreover, the residence time of the viscous material in the in-line heater or heated hose may not be sufficient to heat a substantial portion of the viscous material to or near the ideal dispensing temperature. The temperature variations caused by the cold junctions and insufficient residence time in the auxiliary heaters may not be capable of being accommodated by the heater at the tip of the dispensing apparatus, thus resulting in an inconsistent dispensing process.
Furthermore, once the viscous material is delivered to the fluid filled chamber in the dispensing apparatus, the material is susceptible to heat loss through the body of the apparatus resulting in additional temperature variations in the viscous material. Again, the heater at the tip of the dispensing apparatus may be inadequate to uniformly heat the viscous material in the fluid-filled chamber at or near the ideal dispensing temperature due to the heat loss while the viscous material is resident in the fluid chamber.
Accordingly, there is a need for an improved apparatus and method for dispensing discrete amounts of viscous material onto a workpiece in a more isothermal manner.